1. Field of the Invention
The present invention relates to radar systems. More specifically, the present invention relates to pulse compression systems for use in radar systems.
2. Description of the Related Art
A radar system transmits a pulse of electromagnetic energy and detects reflections of same from objects to effect object or target discrimination. The pulse is often chirped, quickly swept over a limited range of frequencies. When received by a matched filter having a dispersive delay line, the stretched or swept pulse is compressed providing most of the energy within a short time interval and thereby facilitating detection and correlation. As the frequency and phase of the transmitted pulse are critical, in general, the transmitted pulse must be coherent. That is, the transmitted pulse must be phase locked to a known reference. This is especially true for W band radar systems operating in the range of 94 gigahertz (Ghz). In this range, it has been difficult to generate a coherent chirped signal inexpensively.
Several approaches have been used to address this problem. For example, one approach involves injection locking. In accordance with this scheme, the transmitter is phase locked to a well-controlled waveform created at a low power level. This signal is tracked by the transmitter as it generates the output pulse. Unfortunately, this approach is quite costly due to the requirement of a very stable chirp signal which must be multiplied up to a higher frequency before amplification and transmission.
Another approach involves pulsing of an impatt diode with a direct current. In response to the current, the diode generates an output signal which may be used as a chirp signal. Unfortunately, the phase and frequency of the output signal is generally not known sufficiently to effect a subsequent compression of the received pulse.
With respect to the compression requirement, surface acoustic wave (SAW) dispersive delay lines have been considered and used in some applications. These devices have a predetermined transfer function such that when the received pulse is matched, it is compressed. If not, the resulting output is useless. Unfortunately, SAW devices are fixed and not adaptive. The transmitter must be tuned to the SAW device. However, transmitter characteristics vary from one unit to the next. Further, these characteristics are phoned to drift in response to contact or changes in operating temperature. Accordingly, matching transmitters to SAW devices has heretofore been exceedingly difficult.
Thus, there is a need in the art for an inexpensive, yet accurate technique for improving the performance of noncoherent pulse compression systems.